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Production
The material commonly known as tungsten carbide is a product of powder metallurgy. Put simply tungsten carbide is a fine powder of pure tungsten carbide held together in cobalt-based binder. This composition can include carbides of titanium, tantalum, niobium or vanadium. Without the cobalt, tungsten carbide would be extremely hard, but would be very brittle and very porous. The soft, more ductile cobalt coats the carbide particles and fills any inter-granular voids. This introduces mechanical strength at the cost of some hardness, making a very practical material.
Manufacture starts with mixing of tungsten carbide powders, with the cobalt and additional carbides The powder’s grain size ranges from sub-micron to between 5 and 6mm. The choice of grain size and distribution has a considerable effect on mechanical properties.
Small wax additions act as binders and lubricants during pressing. The waxed powders are shaped into solid, near net-shape compacts. These are then heated (to around 600-900°C) to vaporise the wax, leaving a ‘pre-sintered’ compact. At these temperatures, the cobalt establishes light bonds, which on cooling give the compact a porous and friable texture, similar to chalk.
In this state the material can, with care, be machined to a final shape, with allowances made for shrinkage. These vary for different compositions, but are generally around 20% by linear measurement, or 50% by volume.
Sintering occurs in high temperature furnaces, either under vacuum or under a hydrogen atmosphere. The temperature is dictated by the grade of the carbide. Generally temperatures range from 1300-1500°C. During sintering the cobalt melts, filling any remaining voids and thoroughly bonding the structure. The compact now exhibits its final high-density and high-hardness properties.
Properties
The properties of various tungsten carbide grades are determined by the composition of elements added and by the grain size. Quality is a matter of tight control over processing time, temperature and atmosphere as well as the complete absence of chemical or physical contamination. Specific figures for particular properties of the various grades of Dymet carbides are always available on request. Some generalisation, however, may be useful and may give some guidelines for design purposes.
Hardness
Tungsten carbide has a very high hardness, therefore specialist sample preparation and testing facilities are required. Vickers Diamond Pyramid Indentation (VPN) is the most effective system, recording hardness in the range of 950Hv for impact resisting grades such as D25, to about 2000Hv for high-finish cutting grades like DF6.
Specific gravity
Specific gravities range from 11.5 to a little over 15, depending on the composition. The lower end of the scale is associated with a high titanium-carbide content, and the upper end of the scale with a very low binder content. To put this in context, this is 1.3 to 1.9 times heavier than mild steel.
Transverse rupture strength
Carbide’s lack of ductility means that tensile strength is difficult to measure with any degree of accuracy. Transverse rupture is a convenient and relatively consistent method of comparing materials. This is calculated by loading the centre of the sample whose ends are supported on two fulcrum edges, rather like a three-point bend test used in polymer testing. Measurements tend to be inversely proportional to hardness, ranging from 180-N/m2. in. for soft, tougher grades down to 80-N/m2 in. for harder more wear resistant grades.
Modulus of elasticity
Tungsten carbide is generally regarded as a non-ductile material, with a Young’s modulus about 2-3 times that of steel. However, this very high stiffness is advantageous in solid carbide tooling, boring bars, precision spindles and other components where rigidity must be maintained under heavy loads.
Thermal properties
Tungsten carbide’s coefficient of linear expansion lies between 5.0 and 6.0x10-6 °C-1, on average about ½ that of steel. An exact value depends on the chemical composition and the working temperature of the material. Tungsten carbide’s mechanical properties can be maintained over extremely large temperature ranges. Hardness and transverse rupture strength, for example, remain relatively unaffected from cryogenic temperatures right up to 800°C-900°C, above the annealing temperatures of most tool steels. However, carbide’s dissimilarity of thermal expansion to other materials becomes significant in composite when temperature fluctuations are significant.
Corrosion resistance
Tungsten carbide is generally accepted as having high corrosion resistance. However, the cobalt binders are vulnerable to certain concentrated acids. It may also be subject to galvanic corrosion, for example, when assembled with copper-bearing materials in the presence of fluids, which act as electrolytes. Such applications are extremely rare, but when they arise it is again the cobalt binder that is vulnerable. Under highly corrosive conditions, the cobalt binder can be replaced with nickel. However, this increase in corrosion resistance is accompanied by a detrimental effect on the mechanical properties. Many corrosive fluids contain abrasives, and tungsten carbides combined resistance to corrosion and abrasion is often the governing factor in choosing this material.
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